Internal-combustion engines without supercharging are known to have a high weight-to-power ratio as well as a high size-to-power ratio.
The power of an internal-combustion engine can be materially increased by its supercharging, i.e. by increasing the pressure of the air supplied to cylinders of the internal-combustion engine.
The internal-combustion engine supercharging can be accomplished with the use of supercharging units providing the compression of the air supplied to the internal-combustion engine. The most promising supercharging unit is the turbo-charger, whose turbine is not mechanically coupled with the internal-combustion engine crankshaft, while it is driving a compressor performing the compression of the air supplied to the internal-combustion engine. The use of these supercharging units will be discussed below.
A power-plant is a combination of an internal-combustion engine with a supercharging unit and other elements which are required to provide the supercharging. Such a power-plant allows the power of the internal-combustion engine included therein to be substantially increased along with a slight increase in its size, weight and cost as compared with non supercharged internal-combustion engine.
The presence of the turbo-charger in the power-plant may, however, lead to an increase in time required to change from a low power level to a higher one, to an abrupt decrease in the maximum torque of the internal-combustion engine shaft as the speed of said shaft is decreased, etc.
The principal disadvantage of a power-plant incorporating an internal-combustion engine with a high level of supercharging produced by a turbo-charger is a reduced range of permissible operating conditions of the internal-combustion engine, resulting from a compressor surging region that diminishes the range of the air flow rate of the compressor and, hence, that of the internal-combustion engine and causes a temperature rise of the exhaust gases at a reduced speed of the internal-combustion engine crankshaft up to the maximum permissible value for the materials of the internal-combustion engine components and assemblies. The most efficient and reliable means for expanding the range of permissible operating conditions of the internal-combustion engine is to provide the power-plant with an auxiliary combustion chamber arranged in the by-pass duct communicating the compressor outlet with the turbine inlet and by-passing the compressed air not used by the internal-combustion engine from the compressor outlet to the turbine inlet through the auxiliary combustion chamber.
This can provide the compressor operation outside the surging region, ensuring the independence of the amount of air supplied by the compressor and the flow rate of the air consumed by the internal-combustion engine.
Furthermore, the auxiliary combustion chamber and the by-pass duct maintain such a compressor outlet pressure that provides the maximum permissible value of exhaust gas temperature under any operating conditions of the internal-combustion engine.